what to compare.. Torque or HP?

[brad black]

last week i could've shown you a graph   oh well.

all engines of the same size will make similar peak torque, just the rpm at which it occurs will vary, and with that the intensity of power delivery.  you accelerate mostly at low to mid rpm, so that's where you need the power on a road bike.  power is torque x rpm, so for more power you need more torque.  for more torque you need more capacity.

the s2r1000 will make you think "wow, this things got some grunt".

the s4rs with make you think "holy sweet mother of jesus" as the front comes up in second and puts you into next week with a smile a mile wide.

whereas the s4r will get you to next week a tad slower.  less air flow potential through the heads, slightly less power, similar torque and at similar rpm.

how much torque and where it occurs defines how it rides pretty much, altho without seeing a curve it can be a bit deceptive.  power is a consequence of that.  generally the bigger the engine the better the streetbike, as you have the grunt to get you going.

if you've ridden an inline 4 600 and had to count time passing as you wait for it to come on then you want a bigger engine.  but, once you've reached 12,000 in first it's all go because from then on you're in the power.  on a drag strip the main difference wil be 60 ft times for instance, with similar power you get similar trap mph even if the time is slower.

if you don't want to rev it get a bigger engine.  if you want to go really fast get a bigger, powerful engine.

[superjohn]

On top of al the great advice so far, you also have to take into account the sort of riding you're likely to be doing. A 4V motor is still going to feel much stronger if you're doing nothing but accelerating and decelerating because of the way the engine makes power. With the wide flat torque curve of a 2V motor, it's going to feel more responsive in traffic and in everyday driving. This was one reason many reviewers noted that the less powerful ST3 was actually easier to ride than the ST4 that preceded it even though the ST4 made a lot more power. The mill in the ST4 was just so peaky it didn't make for a "casual" sort of ride.

Then again, if you don't want casual 

[brad black]

the 2v motors generally have narrower, more rounded torque curves than the 4v motors and a lower peak torque output for a given capacity.  the st3 makes about 10% more torque than the st4 because it's about 10% bigger, but power wise it makes about 7% less.  all std the st4s makes similar peak power to the st4, but has the midrange of the st3, as they're the same capacity.  comparatively the st4 feels slow, but with better top end and therefore like it needs to be revved harder to work due to the comparative shape and lower peak of the torque curve - the st3 curve is much more like a 2v curve, but with a broader spread.  the st4 has the older, longer duration inlet cams which at std settings would exaggerate that.

play with the cam timing tho and you can change the way they feel quite markedly.  you can even make a 748 feel like it has no top end.  i played with a demo 1000ss and varied the cam timing by 15 degrees from 106 to 121 inlet centerline.  made 4% less peak torque at 121, but felt much different in terms of delivery and torque curve width.  121 had 3 less hp under 7,000, the curves crossed at 7,500 and it had 5 more at 8,500. it felt slower, but much more top endy, whereas 106 felt like an old long manifold carby motor with lots of midrange, but no top end and sort of boring in a straight line at least.

you can make an st4 feel like that too, and make an st3 feel like it has to be revved hard.

[Capo]

Here is a dyno of my S4R, the dashed line was 'out of the box' the solid line was after the mods. There is a real kick in the ass at 6.5k.

[Cucciolo]

Here is a dyno of my S4R, the dashed line was 'out of the box' the solid line was after the mods. There is a real kick in the ass at 6.5k.

WOW... that is a huge difference in performance from stock.. what mods did you do.. and if you were to choose one only, which one has the grater impact on the graph?

[Capo]

Thats the DP full 50mm Termis kit that includes SPS cams, also the heads were flowed by John Hackett. It was all done at the same time so I couldn't tell you what contributes to how much.
The engine is back with him now for further mods.